There are many metalworking processes that improve the dimensional accuracy of a metal product. There are also processes that improve the smoothness of a metal surface. These methods include the traditional metal cutting techniques known as turning, milling, honing, and grinding for dimensional accuracy, and polishing, lapping, and burnishing for surface smoothness.
Of particular interest for both dimensional and smoothness quality is burnishing, and an extension of this technique called bearingizing. Most commonly, these two techniques are applied to cylindrical surfaces, such as the surfaces of a cylinder or tube.
Burnishing, in its most common form, uses many conical hard steel rollers to roll the product surface smooth. A typical form of a burnishing tool is schematically illustrated in FIGS. 1A-E for inner diameters and FIGS. 2A-E for outer diameters. Throughout the prior art figures, the following elements are labeled as follows: workpiece-W; worked surface-S; mandrel-M; mandrel force-P; rollers-R; roller cage-C; and support ring-L. Downward pressure is exerted on the mandrel M as indicated by the arrows P by any suitable means such as a spring or pneumatic or hydraulic pressure, as the mandrel M is rotated. The mating conical surfaces of the mandrel M and rollers R result in the rollers R being pressed against the worked surface S of the workpiece W (which is held stationary), resulting in densification and hardening of the surface S. The design is such that a burnishing tool will accommodate a substantial range of component diameters, for example a range of 0.040 inches (2 mm) in diameter of the surface S.
Bearingizing differs from burnishing in that it uses cylindrical rollers, and instead of only rolling the surface, the rollers also impact indent and thereby "peen" the surface. This is illustrated in FIGS. 3A-E and is accomplished with a parallel flat-sided mandrel M which is rotated inside of the roller cage C so that its points T (FIGS. 3A-E) impact the rollers R The rollers may be spring biased by the cage C against the outer surface of the mandrel M or not, but in any event are permitted sufficient radial movement by the cage C so as to impact the surface S when a point T passes by a roller R The result is that greater local pressure is applied to the metal surface S, and so the process is faster. It is also more accurate than burnishing. However, the bearingizing tools will only accommodate a diametral range of 0.004 inches (0.1 mm) and only applies a 0.002 inch (0.05 mm) change in diameter.
Recent developments in powder metallurgy require a different tool to achieve a different end result. In the case of powder metallurgy, great benefit in component functional properties can be gained by raising the density of the surface layer which rubs, rolls, or bounces against another component. In particular, bearing surfaces which must withstand rolling and rubbing stresses suffer from a failure mode called "rolling contact fatigue". This failure mechanism can be minimized by raising the surface layer density of a powder metallurgy bearing surface to near full density. In order to achieve this, a substantial amount of surface metal must be radially compressed. While this can be accomplished to some degree by roller burnishing, the hammering or peening action in bearingizing is more effective, and can produce a deeper layer of densification, which is beneficial in resisting higher surface stresses. It also can produce compressive residual stresses in the densified surface which further raises resistance to rolling contact fatigue.
The limited radial depth of movement of known bearingizing tools cannot accommodate this need. It would be necessary, for example, to use five separate bearingizing units to achieve a 0.010 inch (0.25 mm) diametral reduction. This is both very costly in equipment, and slow in process to the point of being impractical. What is needed for surface densification of powder metallurgy product, therefore, is a bearingizing tool with a practical range of diametral reduction.